Protective circuits for electromagnets



April 21, 1953 J. G. ELWOOD 2,635,079

PROTECTIVE CIRCUITS FOR ELECTROMAGNETS Filed NOV. 50, 1950 9 Q [mentor JUL/U5 Ln 000,

Patented Apr. 21, 1953 PROTECTIVE CIRCUITS FOR ELECTROMAGNETS Julius G. Elwood, Gary, Ind., assignor to United- States- Steel Corporation,

Jersey a corporation of New Application November 30, 1950, Serial No. 198,392

4 Claims.

This invention relates to improved protective circuits for electromagnets.

In the operation of electromagnets it is known that, when a magnet is tie-energized to drop its load, some residual magnetism tends to remain, and also that the fiux decay creates an undesirable reverse voltage surge in the coils. Consequ-ently the usual practice is to equip electromag nets with a drop circuit which energizes them briefly in the reverse direction to dissipate the residual magnetism and which also absorbs the reverse voltage surge. The drop circuit comprises conductors and contactors for directing line current through the magnet coil in the reverse direction, but the conductors contain resistances, since only a small current is needed compared to the current which the lifting circuit supplies. These same resistances also absorb some of the reverse voltage surge and prevent its damaging any of the parts. Such a drop circuit satisfactorily accomplishes its purpose as long as the conductors, resistances and contactors are in proper working order. However, if there is any break, such as a burned-out resistance, the drop circuitfails to function and the reverse voltage surges are applied to the lifting circuit. A small number of such surges can burn the lifting circuit contacts and can carbonize the insulation and thus cause serious damage.

An object of the present invention is to provide improved protective circuits which absorb reverse voltage surges from clectroma-gnets in the event of a break in the drop circuit.

A further object of the invention is to provide improved protective circuits which comprise a resistance connected across the coil of an electromagnet for absorbing reverse voltage surges and a control relay therefor connected to the drop circuit for cutting out this resistance automatically except when the drop circuit contains a break.

Afurther object of the invention is to provide an improved type of protective circuit which is suitable for magnets having various commercially available lifting and drop circuits.

In accomplishing these and other objects of the invention, I have provided improved details of structure, a preferred form of which is shown in the accompanying drawing, in which:

Figure 1 is a simplified diagram which illustrates the principles of an electromagnet lifting circuit, drop circuit, and the protective circuit of the present invention; and

Figure 2 is a wiring, diagram which shows in detail how this protective circuit can be applied to one commercially available form of lifting and drop circuit; Y

Figure 1 shows an electromagnet MG, the terminals Ti and T2 of which are connected" to positive and negative lines through sets of liftingcontacts L1 and L2 respectively. When both these sets of contacts are closed, the lifting circuit is completed and the magnet is energized for lifting a load. The second terminal T2 of the magnet also is connected to the positive line through a set of drop contacts D1 and a resistance R1 (on the order of 5 to 10 ohms). Likewise the first terminal T1 of the magnet is connected to the negative line viaanother set of drop contacts D2 and another resistance R2, similar to R1. When the drop contacts D1 and D2 are closed as th lifting contacts L1 and Lo, open, the magnet is energized slightly in the reverse direction for overcoming its residual magnetism and thus dropping its load. The resistances R1 and R2 absorb part of the reverse voltage surge produc :l y the flux decay as the lifting contacts pen. The simplified diagram does not show the control circuit for operating the lifting and drop contacts, but it can be any standard or desired circuit, such as one of those hereinafter fully described. The control circuit automatically closes the drop contacts D1 and D2 when it opens the lifting contacts L1 and L2, and it also absorbs part of the reverse voltage surge.

Lifting and drop circuits which embody these fundamentals are well knownand they operate satisfactorily as long asth'ey remain intact. However, when there is a br al; in the drop circuit, such as a burned-out resistance R1 or R2, the drop circuit neither energizes the magnet in the reverse direction nor absorbs the reverse voltage surge. Failure to energize the magnet is not especially serious, since it merely causes some particles to cling to the magnet. However, failure to absorb the voltage surge sends this surge through the lifting circuit, where as already explained it can cause serious damage.

The protective circuit of the present invention comprises a resistance RE (on the order of 10 to 15 ohms), a set of normally closed contacts S2 connected in series-with said resistance across magnet MG, and an operating relay coil S connected across the drop circuit outside the drop contacts Di and D2. The protective circuit also has two more sets of normally open contacts S1 and D3. Contacts S1 are situated between contacts D1 and the magnet, and contacts D3 are in series with coil S.

When the drop circuit is working properly, contacts D3 close with contacts D1 and D2 and energize coil S. Thereupon the normally open contacts Sr close and the normally closed contacts S2 open. Closing of contacts S1 completes the drop circuit to the magnet in the usual way. Opening of contacts S2 automatically cuts out resistance Rs. When there is a break in the drop circuit, closing of the drop circuit contacts fails to energize coil S and the normally closed contacts S2 remain closed. Resistance RE remains connected and absorbs the reverse voltage surge from the magnet and thus fully protects the lifting circuit from damage. Contacts S1 remain open and prevent this surge from energizing coil S.

Figure 2 shows in detail how the protective circuit just described can be applied to one form of commercially available lifting and drop circuit. Figure 2 shows a magnet 10, positive and negative direct current lines l2 and i3 and a master switch 14, Ma. The magnet has terminals Illa and lllb. The lifting circuit includes a relay L which has the usual four sets of normally open contacts 15, it, I? and i8 and the usual one set of normally closed contacts 19. Contacts i5 and I6 correspond with contacts L1 and L2 of Figure l. The drop circuit includes a relay D which has the usual two sets of normally open contacts 2e and 2% and one additional set of normally open contacts 22 for the protective circuit. These contacts correspond respectively with contacts D1, D2 and D3 of Figure 1. The protective circuit includes a relay S which has one set of normally open contacts 23 and one set of normally closed contacts 2t. These contacts correspond respectively with contacts S1 and S2 of Figure 1. r

A conductor 25 connects the positive line 52 with one end of the coil of relay L and contains a normally open lift switch 25. A conductor '21 connects the other end of this coil with the negative line it and is conncted en route with one contact or" the normally open set it of relay L. A conductor 28 connects the positive line 52 with one contact of the normally open set it of relay L. A conductor 28 connects the other contact of this set with terminal lilo of magnet iii. A conduotor 39 connects terminal it?) thereof with one contact of the normally open set it of relay L.

A conductor 3! connects the other contact of this set with the negative line it. With master switch M, Ma closed, closing of switch 26 energizes the coil of relay L from line i2, via conduotor 25, the relay coil, and conductor 21 to line It. Contacts it and It thereupon close and complete the lifting circuit to the magnet i0.

The current path is from line l2, conductor 28,

contacts i5, conductor 29, magnet is, conductor 30, contacts is and conductor 3! to line it.

A conductor 32 connects the positive line it (via conductor with one contact of the normally open set ll of relay'L and contains a normally closed drop switch 33. The drop and lift switches are interconnected so that when either closes the other opens, but when the drop switch closes, the lift circuit remains energized for an instant. A conductor 3 connects the other contact of set i? with one end or" the coil of relay D. A conductor 35 connects the other end of this coil with one contact of the normally open set [B of relay L. Conductor 2?, hereinbefore referred to, connects the other contact of this set with line it. A conductor 36 connects line 32 (via conductor 25) with one contact of each of the normally open sets 20 and 22 of relay D and contains a resistance 3? that corresponds with resistance R1 of Figure l. A conduotor 33 connects the other contact of set 20 with one contact of the normally open set 23 of relay S. A conductor 39 connects the other contact of set 23 with terminal lilb of the magnet and is connected en route to one contact of the normally closed set 24 of relay S. A conductor 4t connects the other terminal Illa of the magnet with one contact or" the normally open set 2! of relay D. A conductor 4| connects the other contact of set 2! with line [3 and contains a resistance 42 that corresponds with resistance R2 of Figure 1.

When drop switch 33 closes and lift switch 25 opens, there is some lag in the operation of relay L. Consequently relay D is energized, the current path being from line 12, via conductors 25 and 32, switch 33, contacts ll (which remain closed momonetarily during the lag), conductor 34, the coil of relay D, conductor 35, contacts it (which likewise remain closed during the lag) and conductor 2? to line l3. Thereupon contacts 20, 2| and 22 close. Closing of contacts 20 and 21 completes the drop circuit to the magnet. The current path is from line i2, via conductors 25 and 36, resistance 33, contacts 20, conduotor 38, contacts 23 (which also close as hereinaiter explained), conductor 3%, the magnet, conductor e5, contacts 25, conductor ii and resistance 42, to line It. By this time relay L is tie-energized and its contacts have returned to their normal position. Opening of contacts i5 and 16 of relay L breaks the lifting circuit to the magnet, so that only the drop circuit is energized. Resistances 3i and 42 limit the current which flows to the magnet.

By connections next described, the reverse voltage surge from the magnet maintains the coil of relay D energized for a period after the contacts of relay L return to their normal positions. A conductor 43 connects magnet terminal ma with one contact of the normally closed set l9 of relay L and contains a high resistance 64 (on the order of 1000 to 3000 ohms). A conductor 45 connects the other contact of set it! with conduotor 35. A conductor 46 connects conductor 34 with magnet terminal I627, and contains a high resistance t? similar to resistance as. Opening of contacts 15 and it creates a flux decay which in turn creates the reverse voltage surge. Part of the resulting current travels from magnet terminal lilb via conductors it and 3t, resistance 41!, the coil of relay D, conductors 35 and 55, contacts It, conductor Q3 and resistance 44 to magnet terminal Ilia. Thus part or this voltage surge is utilized for holding in the drop circuit. Another part of this surge is absorbed in resistances 3? and -22, the same as in resistances R1 and R2 of Figure 1. A conductor 48 connects condoctors 45 and 3 3 and contains a rheostat 19 and a resistance 59 (on the order of 500 ohms and 50 ohms respectively). surge passes and the coil of relay D is tie-energized, it has its own flux decay which creates a further voltage surge. The current from this latter surge moves from the coil of relay D via conductors 34 and 48, resistance 50, rheostat 49, and conductors t5 and 35 back to said coil and tends to hold the contacts 20, 25 and 22 in their energized positions.

The circuits thus far described, except for the connection through relay S and contacts 22, are known and per se are not part of the present invention. As already explained, such circuits operate satisfactorily unless a break occurs in the drop circuit. Such a break would send the reverse voltage surges from the magnet through When the reverse voltageconductors 29 and to contacts l5 and [6. The current from such surges would are across these contacts while they are opening, and repeated arcing ultimately would burn these contacts, as well as carbonizing the insulation.

In accordance with the present invention, a conductor 5! connects one end of the coil of relay S with conductor 4! between contacts 2! and resistance 42. A conductor 52 connects the other end of said coil with one contact of the normally open set 22. As already described the other contact of set 22 is connected to conductor and thence to line 12. Energizing of relay D and closing of its contacts 22 energizes relay S. Normally the current path is from line l2 via conductors 25 and 36, resistance 31, contacts 22, conductor 52, the coil of relay S, conductors 5| and 4|, and resistance 42, to line l3. Contacts 23 of this relay thereupon close and complete the drop circuit, as previously mentioned but not explained.

As also previously mentioned, conductor 39 connects magnet terminal [6b with one contact of the normally closed set 24 of relay S. A conductor 53 connects the other contact of set 24 with magnet terminal Ida and contains a resistance 54 that corresponds with resistance RE of Figure 1. When relay S is energized, its contacts 24 open and break the circuit through resistance 54. Therefore, if the drop circuit is in proper working order, there is no need for resistance 54 and it becomes disconnected.

Assume now that there is a break in the drop circuit, such as a burned-out resistance 31 or 42. Closing of drop switch 33 and simultaneous opening of lift switch 26 energizes the coil of relay D and de-energizes the coil of relay L as before. However, closing of contacts 20 and 2| does not complete the drop circuit since it is broken elsewhere. Relay D becomes de-energized after an instant, as contacts I! and I8 of relay L very soon open. Relay S has not become energized and its contacts 24 remain closed. The resistance 54 safely absorbs the reverse voltage surge from the magnet. The current path is from magnet terminal lob, conductor 39, contacts 24, resistance 54 and conductor 53 to magnet terminal Hid. Contacts 23 of relay S remain open and prevent any tendency for this surge to energize relay S.

While I have shown in detail how the protective circuit of the present invention can be applied to one commercially available form of lifting and drop circuits, it is apparent that the same protective circuit is applicable to other commercially available forms, and it is possible other modifications may arise. Therefore I do not wish to be limited by the disclosure set forth, but only by the scope of the appended claims.

I claim:

1. In an electromagnet having lifting and drop circuits, at first relay which when energized is adapted to close said lifting circuit, and a second relay which after said first relay is deenergized, is adapted to be energized solely from flux decay and when energized is adapted to close said drop circuit, the combination therewith of a protective circuit comprising a resistance connected across the magnet and capable of absorbing the voltage surge produced by flux decay when said lifting circuit is broken, and a third relay having its coil connected in said drop circuit, said second relay having contacts for closing a circuit to said third relay when said drop circuit is closed, said third relay having normally closed contacts in series with said resistance for disconnecting said resistance when said drop circuit is energized and working properly.

2. In an electromagnet having lifting and drop circuits, a first relay which when energized is adapted to close said lifting circuit, and a second relay which, after said first relay is deenergized, is adapted to be energized solely from flux decay and when energized is adapted to close said drop circuit, the combination therewith of a protective circuit comprising a resistance connected across the magnet and capable of absorbing the voltage surge produced by fiux decay when said lifting circuit is broken, and a third relay having normally closed contacts in series with said resistance, said second relay having normally open contacts in series with the coil of said third relay, which latter contacts on closing are adapted to complete a current path through the coil of said third relay and thus disconnect said resistance as long as said drop circuit is in working order.

3. In an electromagnet having lifting and drop circuits, a first relay which when energized is adapted to close said lifting circuit, and a second relay which, after said first relay is deenergized, is adapted to be energized solely from flux decay and when energized is adapted to close said drop circuit, the combination therewith of a protective circuit comprising a resistance connected across the magnet and capable of absorbing the voltage surge produced by flux decay when said lifting circuit is broken, and a third relay having normally closed contacts in series with said resistance and normally open contacts in said drop circuit, said second relay having normally open contacts in series with the coil of said third relay, which last named contacts on closing are adapted to complete a current path through the coil of said third relay and thus disconnect said resistance by opening the normally closed contacts of said third relay and complete the drop circuit by closing the normally open contacts of said third relay.

4. In an electromagnet having lifting and drop circuits, a first relay which when energized is adapted to close said lifting circuit, and a second relay which, after said first relay is deenergized, is adapted to be energized from fiux decay and has contacts adapted to close said drop circuit, the combination therewith of a protective circuit comprising a resistance connected across the magnet and capable of absorbing the voltage surge produced by fiux decay when said lifting circuit is broken, a set of normally closed contacts in series with said resistance, and operating mechanism for said last named contacts connected with said second relay for opening these contacts and cutting out said resistance concurrent with energization of said second relay as long as said drop circuit works properly, whereby said protective circuit functions only if said drop circuit is disrupted.

JULIUS G. ELWOOD.

References Cited in the file of this patent UNITED STATES PATENTS Number July 2, 1940 

